In the modern atomic model, electrons form an electron cloud. Anelectron cloudis an area around an atomic nucleus where an electron is most likely to be located. Imagine taking a time-lapse photograph of bees around a hive. You might see a blurry cloud. The cloud might be denser near the hive than farther away because the bees spend more time near the hive.

In a similar way, electrons constantly move around the nucleus. It is impossible to know both the speed and exact location of an electron at a given moment in time. Instead, scientists only can predict the likelihood that an electron is in a particular location. The electron cloud shown in Figure 10is mostly empty space but represents the likelihood of finding an electron in a given area. The darker areas represent areas where electrons are more likely to be.

ATOM FACTSAtoms are made up of electrons, protons and neutrons.

Air fills most of an atom.

The nucleus of the atom is made of protons and neutronsat the center of an electron cloud.

The Parts of the Atom

If you could see inside any atom, you probably would see the same thing—empty space surrounding a very tiny nucleus.

A look inside the nucleus would reveal positively charged protons and neutral neutrons.Negatively charged electrons would be whizzing by in the empty space around the nucleus.

Table 1compares the properties of protons, neutrons, and electrons. Protons and neutrons have about the same mass. The mass of electrons is much smaller than the mass of protons or neutrons. That means most of the mass of an atom is found in the nucleus. In this lesson, you will learn that, while all atoms contain protons, neutrons, and electrons, the numbers of these particles are different for different types of atoms.

Table 1Properties of Protons, Neutrons, and Electrons

Different Elements—Different Numbers of Protons

Look at the periodic table in the Reference Handbook. Notice that there are more than 115 different elements. Recall that an element is a substance made from atoms that all have the same number of protons. For example, the element carbon is made from atoms that all have six protons. Likewise, all atoms that have six protons are carbon atoms. The number of protons in an atom of an element is the element’satomic number.The atomic number is the whole number listed with each element on the periodic table.

What makes an atom of one element different from an atom of another element? Atoms of different elements contain different numbers of protons. For example, oxygen atoms contain eight protons; nitrogen atoms contain seven protons. Different elements have different atomic numbers. Figure 1shows some common elements and their atomic numbers.

Neutral atoms of different elements also have different numbers of electrons. In a neutral atom, the number of electrons equals the number of protons. Therefore, the number of positive charges equals the number of negative charges.

1.Reading CheckWhat two numbers can be used to identify an element?

the atomic number and the number of protons

Figure 1Atoms of different elements contain different numbers of protons.

Visual CheckExplain the difference between an oxygen atom and a carbon atom.

Students may need to be reminded that although isotopes of an element have different numbers of neutrons, they have the same number of protons. Use these questions to guide students in understanding these concepts.

Guiding Questions

How are isotopes of an element alike?

They have the same number of protons.

Explain how the number of neutrons affects the mass of an atom.

Atoms with more neutrons have more mass than do atoms with fewer neutrons.

Protons, Neutrons, and Mass Number

The mass number is the sum of the protons and neutrons in the nucleus of an atom. Isotopes of an element have different mass numbers because they have different numbers of neutrons. These questions provide an opportunity for students to practice calculations involving mass numbers of isotopes.

Guiding Questions

Reading CheckHow do two different isotopes of the same element differ?

Two different isotopes of an element have different numbers of neutrons, but the same number of protons.

How many protons, neutrons, and electrons are in neutral atoms of hydrogen-1, hydrogen-2, and hydrogen-3?

Some students may find it difficult to understand the concept of average atomic mass because they may not have been exposed to weighted averages. Use these questions to help students understand this sometimes confusing concept. Showing students examples of how average atomic mass is calculated is often the best way to reinforce this concept.

Guiding Questions

Why is it necessary to calculate the average atomic mass?

Because a given element has multiple isotopes.

Reading CheckWhat does the term weighted average mean?

Weighted average means that some data points contribute more to the average, or are more abundant, than others.

How is the average atomic mass of an element calculated?

The abundance of each isotope is changed into a decimal form. The mass of each isotope is multiplied by its corresponding decimal percentage. All the values are adding together to determine the average atomic mass.

Differentiated Instruction

Describing IsotopesCreate a sample of “beanium” by mixing several types of dry beans, such as pinto, navy, kidney, and lima. Give each pair of students a handful containing the mixture. Instruct students to pretend that each type of bean is an isotope of a new element called “beanium.” Tell them to describe how the isotopes are alike and how they are different.

Real-World Science

Isotopic AnalysisIsotopic analysis involves determining the proportion of the isotopes in a sample. Isotopic analysis is used in various fields such as archaeology, space science, forensics, food science, and medicine. It is used to identify the origins of meteorites, discriminate among bullet types, determine the origins of cotton fiber used to make counterfeit money, detect alterations of food, distinguish types of wines from imitations, and to verify the authenticity of organically grown crops.

Teacher Demo

Calculating GradesCompare the method of calculating average atomic mass to the method of calculating grades. On the board or chart paper, show students an example of how grades are calculated, using actual assignment weights from your class. Have students do the calculations for the example. Explain to students that, in the example, the assignment weights are analogous to the percentage of each isotope, whereas the average grade for each category is analogous to the mass number.

You have read that atoms of the same element have the same numbers of protons. However, atoms of the same element can have different numbers of neutrons.

For example, carbon atoms all have six protons, but some carbon atoms have six neutrons, some have seven neutrons, and some have eight neutrons.

These three different types of carbon atoms, shown in Table 2,are called isotopes.

Isotopesare atoms of the same element that have different numbers of neutrons. Most elements have several isotopes.

Table 2Naturally Occurring Isotopes of Carbon

Protons, Neutrons, and Mass Number

Themass numberof an atom is the sum of the number of protons and neutrons in an atom.This is shown in the following equation.

Mass number = number of protons + number of neutrons

Any one of these three quantities can be determined if you know the value of the other two quantities. For example, to determine the mass number of an atom, you must know the number of neutrons and the number of protons in the atom.

The mass numbers of the isotopes of carbon are shown in Table 2.An isotope often is written with the element name followed by the mass number. Using this method, the isotopes of carbon are written carbon-12, carbon-13, and carbon-14.

1.Reading CheckHow do two different isotopes of the same element differ?

Two different isotopes of an element have different numbers of neutrons, but the same number of protons.

Average Atomic Mass

You might have noticed that the periodic table does not list mass numbers or the numbers of neutrons. This is because a given element can have several isotopes. However, you might notice that there is a decimal number listed with most elements, as shown in Figure 2.This decimal number is the average atomic mass of the element. The average atomic massof an element is the average mass of the element’s isotopes, weighted according to the abundance of each isotope.

Table 2shows the three isotopes of carbon. The average atomic mass of carbon is 12.01. Why isn’t the average atomic mass 13? After all, the average of the mass numbers 12, 13, and 14 is 13.

The average atomic mass is weighted based on each isotope’s abundance—how much of each isotope is present on Earth. Almost 99 percent of Earth’s carbon is carbon-12. That is why the average atomic mass is close to 12.

2.Reading CheckWhat does the term weighted averagemean?

Weighted average means that some data points contribute more to the average, or are more abundant, than others.

Figure 2The element carbon has several isotopes. The decimal number 12.01 is the average atomic mass of these isotopes.

Math Skills: Use Percentages

You can calculate the average atomic mass of an element if you know the percentage of each isotope in the element. Lithium (Li) contains 7.5% Li-6 and 92.5% Li-7. What is the average atomic mass of Li?

What happens to a neutral atom if it gains or loses electrons? Recall that a neutral atom has no overall charge. This is because it contains equal numbers of positively charged protons and negatively charged electrons. When electrons are added to or removed from an atom, that atom becomes an ion. Anionis an atom that is no longer neutral because it has gained or lost electrons.An ion can be positively or negatively charged depending on whether it has lost or gained electrons.

Positive Ions

When a neutral atom loses one or more electrons, it has more protons than electrons. As a result, it has a positive charge. An atom with a positive charge is called a positive ion.A positive ion is represented by the element’s symbol followed by a superscript plus sign (+). For example, Figure 6shows how sodium (Na) becomes a positive sodium ion (Na+).

Figure 6An ion is formed when a neutral atom gains or loses an electron.

Negative Ions

When a neutral atom gains one or more electrons, it now has more electrons than protons. As a result, the atom has a negative charge. An atom with a negative charge is called a negative ion. A negative ion is represented by the element’s symbol followed by a superscript negative sign (−). Figure 6shows how fluorine (F) becomes a fluoride ion (F−).

1.Key Concept CheckHow does a neutral atom change when its number of protons, electrons, or neutrons changes?

If the number of protons changes, the atom changes to a new element. If the number of electrons changes, the atom becomes an ion. If the number of neutrons changes, the atom becomes an isotope.

Lesson Review

Visual Summary

Different elements contain different numbers of protons.

Two isotopes of a given element contain different numbers of neutrons.

When a neutral atom gains or loses an electron, it becomes an ion.

1.All atoms of the same element have the same number of protons.

2.Atoms of one element cannot be changed into atoms of another element.

2.Nuclear decay occurs when an unstable atomic nucleus changes into another nucleus by emitting __________.

radiation

3.Describehow two isotopes of nitrogen differ from two nitrogen ions.

Two different isotopes of nitrogen have the same numbers of protons but different numbers of neutrons. Two different ions of nitrogen have the same numbers of protons but different numbers of electrons.

Understand Key Concepts

4.An element’s average atomic mass is calculated using the masses of its

A.electrons.

B.isotopes.

C.neutrons.

D.protons.

5.Compare and contrastoxygen-16 and oxygen-17.

They have the same numbers of protons but different numbers of neutrons.

6.Showwhat happens to the electrons of a neutral calcium atom (Ca) when it is changed into a calcium ion (Ca2+).

Calcium loses two electrons.

7.If an ion contains 10 electrons, 12 protons, and 13 neutrons, what is the ion’s charge?

A.2−

B.1−

C.2+

D.3+

8.How many neutrons does iron-59 have?

A.30

B.33

C.56

D.59

9.Which determines the identity of an element?

A.its mass number

B.the charge of the atom

C.the number of its neutrons

D.the number of its protons

10.The figure below shows which of the following?

A.two different elements

B.two different ions

C.two different isotopes

D.two different protons

Interpret Graphics

11.ContrastCopy and fill in this graphic organizer to contrast how different elements, isotopes, and ions are produced.

Critical Thinking

12.ConsiderFind two neighboring elements on the periodic table whose positions would be reversed if they were arranged by atomic mass instead of atomic number.

There are five instances where the elements would be reversed if the periodic table were arranged by atomic mass. Co and Ni, Te and I, U and Np, Fm and Es, Md and No

13.InferCan an isotope also be an ion?

All atoms of a given element are isotopes. For example, all oxygen atoms are oxygen-15, oxygen-16, or oxygen-17. These are all isotopes. The formation of an ion doesn’t change the number of neutrons. Therefore, an ion of oxygen will have 15, 16, or 17 neutrons. If all atoms are isotopes, then all ions are isotopes too.

14.Summarizehow radioactive decay can produce new elements.

During radioactive decay, the nucleus of the atom is transmuted. That means that the number of protons, neutrons, and/or electrons is changed. Since the number of protons is changed, that means that the atomic number is changed. A change in atomic number corresponds to a change in the type of element present.

15.HypothesizeWhat might happen if a negatively charged ion comes into contact with a positively charged ion?

Negative charges attract positive charges. If the charges are equal in magnitude, the ions would attract each other to create a neutral molecule.

16.InferWhy isn’t mass number listed with each element on the periodic table?

A given element can have more than one isotope and each isotope has a unique mass number. It wouldn’t make sense to only select one of these mass numbers to list and there isn’t enough room to list them all.

17.ExplainHow is the average atomic mass calculated?

Average atomic mass is calculated by first multiplying the atomic mass of each isotope by its respective percent isotopic abundance in nature and then adding these sums together.

18.InferOxygen has three stable isotopes.

What can you determine about the average atomic mass of oxygen without calculating it?

The average atomic mass of oxygen is approximately 16 because oxygen-16 atoms make up over 99.7% of the atoms in nature.

19.Describethe current model of the atom. Explain the size of atoms. Also explain the charge, the location, and the size and mass of protons, neutrons, and electrons.

The current model of the atom is a tiny nucleus that contains positively charged protons and neutral neutrons surrounded by an electron cloud that contains negatively charged electrons. Atoms are so small that they can only be seen by special microscopes called Scanning Tunneling Microscopes. Protons and neutrons are about the same mass but the mass of electrons is much smaller than protons and neutrons.

Math Skills

Use Percentages

20.A sample of copper (Cu) contains 69.17% Cu-63. The remaining copper atoms are Cu-65. What I is the average atomic mass of copper?